1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device having a deep impurity diffusion area, such as a well area, in a semiconductor substrate, which can enhance the gettering of a metal impurity in the semiconductor substrate during a thermal diffusion step of a wafer process.
2. Description of the Related Art
In order to getter, for example, a metal impurity during the manufacturing of a conventional semiconductor device, use is made of a phosphorus getter containing phosphor glass (P.sub.2 O.sub.5) or intrinsic gettering method (IG).
In the IG method, microdefects are formed in a wafer, without using a separate operation, giving the wafer a getting capability.
A silicon crystal which is grown in a CZ (Czochralski) method is usually employed for an LSI element substrate. In recent times, use is also made of an Si crystal grown by a magnetic application type CZ (MCZ) method. The wafer obtained by the above method as grown from a crucible contains supersaturated oxygen extracted and from the atmosphere during crystal growth. Such excess oxygen in a crystal reacts with a portion of the silicon in a thermal process to form an oxide precipitate. Its value change is about twice, and the excess silicon atoms are emitted as self-interstitials. And the excess silicon atoms form dislocations and stacking faults. In general, these oxide precipitate dislocations and stacking faults are called BMD (Bulk Microdefect). The BMD functions as the center of gettering and is capable of gettering, for example, a metal impurity.
The IG method utilizes the gettering capability of the BMD. That is, the IG method achieves a gettering function by out-diffusing oxygen by a heat treatment near the surface of a wafer element formation area to form a denuded zone (DZ) of a predetermined width and to distribute a high-density BMD in an internal region where no element is to be formed.
The IG method will be explained below in connection with a complementary MOS having a well area by referring to FIGS. 1A to 1C.
FIGS. 1A to 1C are cross-sectional views showing the main behavior of oxygen in one form of model. In the case of a complementary type semiconductor device, in order to form a deep P- or N- well in the initial phase of the manufacturing process, an impurity is thermally diffused in the surface of wafer 1 for hours at a temperature of over 1100.degree. C. Of the oxygen in the wafer indicated by small dashes, surface layer oxygen is out-diffused during this step to form defect-free layer 2 (DZ of a predetermined width) in the surface portion of the substrate as shown in FIG. 1A. During the steps of forming a nitride film by a vacuum CVD method, the wafer is heat treated at 600.degree. to 800.degree. C. to form oxygen precipitate nuclei (X in FIG. 1B) in intermediate layer 3 in the wafer as shown in FIG. 1B. During the steps of forming a field oxide film, the wafer is heat treated at about 1000.degree. C. and the nuclei allow precipitates to be grown thereabout, forming gettering zone 4 where BMDs are distributed in high density as indicated by marks in FIG. 1C.
The generation of oxygen precipitates and hence the BMD is largely varied, depending upon not only the heat history but also the carbon density, pulling condition during the growth of a crystal, and so on. At the present time, a possible nucleus of the BMD is regarded as being an oxygen microdeposit upon the pulling of the crystal and, those nuclei, exceeding their critical size, are grown into BMDs. The critical size differs, depending upon the temperature, and the lower the temperature, the smaller the precipitate is grown from the nucleus. The precipitates of varying size and number are dispersed within the wafer.
In the semiconductor device manufactured using the IG method, an element is formed in the DZ, but the conventional technique encounters a problem that the electrical characteristics of elements become poor due to the presence of crystal defects near the wafer surface layer.
Generally, diffusion occurs in the well at a temperature as high as over 1100.degree. C. and the solid solution limitation of the oxygen becomes higher, and the critical size of the precipitate nucleus becomes larger. In view of the above, the behavior of the oxygen near the wafer surface layer is regarded as being an "out-diffusion", but it has been found that, if the precipitate nucleus exceeds the critical size at a prevailing temperature during the diffusion of the well, a lower density of defects are grown into BMDs at the wafer surface layer.
FIG. 2 shows a relation of the surface density (number/cm.sup.2) of the BMD to the depth (.mu.m) from the wafer surface which is obtained from a wafer of an oxygen concentration 1.7.times.10.sup.18 atoms/cm.sup.3 --a value obtained by finding the absorption coefficient .alpha. (cm.sup.-1) of an infrared radiation with a wave number of 1106 cm.sup.-1 and calculating an equation .alpha..times.4.81.times.10.sup.17 atoms/cm.sup.3 --by, subsequent to performing a heat treatment in an N.sub.2 atmosphere at 1200.degree. C. for 3 hours, carrying out a heat treatment in the N.sub.2 atmosphere at 1000.degree. C. for 20 hours. FIG. 3 is a graph showing a relation of an oxygen concentration (atoms/cm.sup.3) to the depth (.mu.m) from the surface of a wafer which was heat treated at 1200.degree. C. for 3 hours as examined by a secondary ion mass spectrometry (SIMS). From FIGS. 2 and 3 it has been found that the out-diffusion of oxygen occurs from a considerably deep position, but that BMDs occur from a relatively shallow position. It has also been found that BMDs generated from a shallow position near the surface of the wafer present dislocation nuclei as generated upon the isolation of elements when an LOCOS method is employed, or the generation center or recombination center of carriers, to adversely affect the electrical characteristics of the elements.